1. Field of the Invention
The present invention relates generally to heating human or animal tissue (hyperthermia) and more particularly to electromagnetic radiation (EMR) apparatus for heating local areas within such living body tissue.
2. Background Information
As is generally known, death, or necrosis, of living tissue cells occurs at temperatures elevated above a normal cell temperature. Further, the death rate of such heated tissue is a function of both the temperature to which it is heated and the duration for which the tissue is held at such temperatures.
It is also well known that the elevation of temperature of living tissue can be produced with electromagnetic energy at frequencies greater than about 10 kHz.
It has been reported that some types of malignant cells may be necrosed by heating them to a temperature which is slightly below the temperature injurious to most normal cells. In addition, some types of malignant cells may be selectively heated and necrosed by hyperthermia techniques because masses of these malignant cells typically have considerably poorer blood flow and thus poorer heat dissipation properties than does the surrounding normal tissue. As a result, when normal tissue containing such malignant masses is heated by EMR (electromagnetic radiation), the resultant temperature of the malignant mass may be substantially above that of surrounding healthy cells.
Although some disagreement exists regarding exact temperatures, most malignant cells have a relatively limited temperature range in which hyperthermia is effective in causing necrosis. Below a thershold temperature of about 41.5.degree. C. (106.7.degree. F.) insubstantial thermal damage occurs even in those types of malignancies which have a greater sensitivity to temperature than do normal cells. In fact, at temperatures just below this threshold, growth of some types of malignancies may be stimulated. At temperatures within or above 43.degree. to 45.degree. C. (109.4.degree. to 113.degree. F.) thermal damage to most normal cells occur. A discussion of hyperthermia in the treatment of cancer is contained in "Physical Hyperthermia and Cancer Therapy" by J. Gordon Short and Paul F. Turner in the Proceedings of the IEEE, Vol. 68, No. 1, January, 1980 herein incorporated by reference.
Typically, EMR heating of body tissue is accomplished by holding an EMR radiator, or applicator, adjacent to, or against, exterior portions of a body, the EMR then penetrating and heating subsurface portions of tissue. However, significant amounts of energy are absorbed by surface or epidermis layers which may have to be cooled in order to prevent damage thereto by overheating.
The amount of penetration, or the depth of which EMR causes effective heating, is dependent upon the frequency of radiation.
For example, in accordance with an article by A. W. Guy, et al, published in proceedings of the IEEE, Vol. 63, No. 1, January, 1974 entitled "Therapeutic Application of Electromagnetic Power", the depth of penetration in the human muscle and fat at 100 MHz is 6.66 cm (2.62 inches) and 60.4 cm (23.8 inches), respectively, while at 915 MHz the depth of penetration is only 3.04 cm (1.2 inches) and 17.7 cm (6.97 inches), respectively.
In general, the lower the EMR frequency, the larger the applicator must be in order to effectively radiate electromagnetic energy into the tissue and, as a result, applicators for radiating electromagnetic energy below one gigahertz tend to be large in size and cumbersome to handle. Additionally, such applicators are not configured to selectively heat tumors of various sizes and shapes located well beneath the surface layers of the body being irradiated. Further, tumors, or other selected areas, shielded by a layer of boney tissue such as a skull, are difficult to effectively heat with externally applied EMR.
Invasive EMR applicators, that is, radiators which can be inserted into body tissue to levels adjacent malignant tumors, or other localized growths, for selective heating thereof, may cause nonuniform heating, or "hot spotting" at or near the surface of such applicators because of nonuniform field distributions. Such unwanted "hot spotting" is more likely to cause serious overheating when such invasive applications are operated at higher power levels in order to heat large localized growths using a single applicator. Such growths may be many times the size of the radiating area of an invasive type applicator.
An example of invasive EMR applicators are disclosed in U.S. Pat. No. 4,448,198 entitled "Invasive Hyperthermia Apparatus and Method" which discloses the application of several invasive type applicators and a method of using the apparatus to effectively heat relatively large localized areas within living body tissue, without significant hot spotting at or about the applicators.
Gammell discloses a similar apparatus in U.S. Pat. No. 4,346,715 which issued Aug. 31, 1982 includes an array of contacting metal electrodes operating the array with radio frequency energy in the range of 500 KHz in a way to cause an isothermal rotating electric field which is confined to the area of the tumor or lesion.
Doss et al. also developed an invasive electrode array described in U.S. Pat. No. 4,016,886 in April of 1977 and which produced a heating field from a fixed current field being established by connecting metal needles directly contacting the tissue. Doss teaches the operation of this apparatus at low radio frequencies below 1 MHz.
German Patent DAS No. 1143937 issued in February, 1963 describes a similar two electrode array. This apparatus also placed metal electrodes in direct contact with the body. With a grounded outer conductor, the heating field may be 3 phased causing microwave energy to have current fields parallel to, and perpendicular to the electrodes. The heating field would be locally positioned at the tips of the electrodes.
German Patent DT No. 2815156 to convert describes an electromagnetic radiating apparatus which is inserted into the tissue to apply HF (high frequency) electromagnetic waves to heat living tissues. This apparatus or probe is adapted to radiate the waves into the tissue like an antenna. Also, multiple arrays of this probe may be used. This device operates at higher frequencies (300-2000 MHz) than the other referenced devices because the inner and outer coax conductors form a monopole type radiating antenna. The earlier references operate as metal contact electrode pairs or arrays which by virtue of their contact to the tissue can induce currents to flow in the tissue. These tissue current fields terminate perpendicular to the inserted electrode surfaces.
The current fields of current are substantially parallel to the radiating shaft as is well known in the sciences for monopoles and dipoles.
Turner disclosed in U.S. Pat. No. 4,448,198 a similar apparatus and method of radiating invasive monopoles was described. This apparatus also operates at high frequencies.
The Oximetrics Corporation has developed a specially designed catheter which has an internal hollow dielectric surrounded by a metal braid which is coated with a removable outer dielectric layer. This enables the clinician to remove segments of the outer dielectric coating to provide selective contact between the metal braid and the surrounding tissues into which the catheter is inserted. This technique applies 500 KHz currents directly into the tissue to cause a local current field between electrode referenced by a presentation at North American Hyperthermia Group by S. D. Prionas, et al, "Interstitial RF Hyperthermia Plus Brachy Therapy of Neoplasma," Stanford University School of Medicine. Clinical use in stimulation of muscle and nerve tissues has shown that this apparatus requires special precautions to shape the waveform or eliminating the current field prior to switching active electrode pairs. These observations have been reported by independent researchers who have shown such stimulation is observed as high as 1 MHz. These researchers have indicated such stimulation is potentially hazardous. Some of the hazardous effects are obturator muscle spasm, cardiac ventricular fibrillation, and pacemaker malfunction referenced by John R. LaCourse, et al., "Effect of High-Frequency Current On Nerve and Muscle Tissue," IEEE transaction BME-32, No. 1, January 1985, pp. 82-86.
This results in the stimulation threshold current increasing montonically with increasing frequency. Therefore, the stimulation current at 50 MHz would be expected to be about 50 times more than the stimulation current at 1 MHz. Thus, for frequencies above 1 MHz tissue, destruction or desiccation probably occurs before the stimulus threshold current can be reached.
The use of frequency of approximately 10 MHz or more would eliminate this potentially hazardous stimulation potential.
The apparatus and methods described by Gammell, Doss, and Oximetrics use the lower frequencies near 500 KHz with contacting metallic electrodes. That of Fritz also uses contacting metal electrodes but with higher frequency microwave fields being radiated.
Convert and Turner methods patented earlier both use radiating electromagnetic waves from each electrode or applicator acting like a monopole antenna.
All these methods require metallic contact to the tissue and sterilization of the electrode or applicator. This would be difficult for repeated use since each has exposed material interfaces which would be difficult to clean after use.
The method and devices of this invention includes the use of a sterile catheter or dielectric tube placed into the tissue to enable the applicator insertion therein. The catheter would either pass completely through the tissue exposing both ends to the outside air, or the catheter would have a closed tip. The use of the high frequency selective capacitive coupling through the catheter is distinctly different from the established prior art.
The procedure to install such catheters is quite common in radiation therapy, where radioactive seeds or wire ribbons are inserted into a number of these catheters to radiate a tumor from inside with ionizing radiation. It has been shown that adding tissue heating to ionizing radiation enhances tumor cell killing and regression. The method of using the standard dielectric catheters (nylon or teflon) for local capacitive heating inside the tumor should minimize treatment costs and improve clinicians acceptance. This method is therefore a significant improvement over the direct contact applicator methods and also different from these methods.
It is the object of the present invention to provide an applicator for inserting into body tissue through a dielectric catheter or sleeve for locally heating these tissues.
It is further the object of the present invention to provide an enlarged diameter metal section for more selective heating in the intended area with reduced heating in the zones of smaller electrode diameters. This is a result of larger capacitance from the enlarged metal section through the catheter to the tissue than the capacitance of the smaller metal sections.
It is still further an object of this invention to provide a system to provide UHF electromagnetic (EM) signals capable of flowing through the catheter wall capacitive impedance.
It is still further an object of this invention to provide an applicator and system which enables the temperature of the electrodes to be measured with or without EM signals being applied to the applicator for the control of the EM power to each applicator to achieve the desired elevation of the tissue temperatures around each applicator.